1. Field of the Invention
This invention relates to vacuum valves in general and, more particularly, to a vacuum reducer valve for producing an output signal at a predetermined vacuum level below an input vacuum signal.
2. Prior Art
Most present day motor vehicles have some sort of a vacuum servo automatically controlling advance or retard setting of the engine distributor braker plate as a function of carburetor spark port vacuum to provide good engine performance, as well as fuel economy during the various operating modes of the engine. With the current concern of noxious gases exhausted into the atmosphere, additional controls have been developed in conjunction with the above vacuum servo in an attempt to reduce the emission of noxious gases exhausted to the atmosphere. One such control is a solonoid operated vacuum control valve used to prevent advancing of the spark before the transmission is shifted into high gear. The solonoid, when energized, prevents carburetor spark port vacuum from advancing the spark by venting the distributor vacuum servo to atmosphere. As the transmission shifts into its high speed drive range, the solonoid coil is deenergized and the valve is effective to provide communication between the distributor vacuum servo and carburetor spark port vacuum. The spark advance then functions normally on carburetor spark port vacuum. Utilizing the solonoid valve to selectively control the distributor vacuum servo causes the engine to operate inefficiently during the idle mode. The reason for this problem is that at idle operation there is zero forward speed as well as high ambient pressure. Also, at idle there is no vacuum at the spark port, therefore, no vacuum spark advance. The advantage of operating with no vacuum spark advance or a retarded spark advance at idle operation is that it reduces the air pollution at idle operation. Such a mode of operation, however, gives rise to another serious problem inherent in the internal combustion engine. Retardation of the spark causes the operating temperature of the engine to rise, and continued operation with a retarded spark causes the temperature to rise to prohibitive levels. By contrast, an advance of the spark, while resulting in more complete combustion and an intended increase in noxious gases being exhausted from the engine, has the salutary effect of reducing the operating temperature of the engine.
To avoid overheating of the internal combustion engine during the idle mode, an engine overheating detection valve, commonly called temperature variable switch (T.V.S. valve), is used in the distributor vacuum spark advance circuit. The T.V.S. valve is a three way valve which normally applies spark port vacuum to the distributor vacuum advance actuator. When an engine overheat condition is encountered, the spark port signal lines are blocked and manifold vacuum is applied to the distributor vacuum actuator. The manifold vacuum, however, averages 31/2 inches of mercury vacuum higher than the spark port vacuum thereby presenting the problem of the need of a vacuum reducer valve in series with the manifold vacuum line to the distributor to avoid excessive spark advance.